path: root/sbin/dhclient/dhclient.8

.\" $OpenBSD: dhclient.8,v 1.38 2019/01/18 07:39:53 krw Exp $
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.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/isc''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.Dd $Mdocdate: January 18 2019 $
.Dt DHCLIENT 8
.Os
.Sh NAME
.Nm dhclient
.Nd Dynamic Host Configuration Protocol (DHCP) client
.Sh SYNOPSIS
.Nm
.Op Fl dnrv
.Op Fl c Ar file
.Op Fl i Ar options
.Op Fl L Ar file
.Op Fl l Ar file
.Ar interface
.Sh DESCRIPTION
The Dynamic Host Configuration Protocol (DHCP) allows hosts on a TCP/IP network
to configure one or more network interfaces based on information collected from
a DHCP server.
DHCP is often used, for example, by cable modem and DSL network
providers to automate network configuration for their customers.
.Pp
Information typically provided via DHCP includes
address and subnet mask for the interface,
default route,
and domain name server.
.Pp
To have
.Ox
configure an interface using DHCP
(or its predecessor, BOOTP)
the
.Nm
utility is used.
.Nm
is run on the command line with the name of the
.Ar interface
to be configured.
.Nm
can also be run at boot time from
.Xr hostname.if 5 ,
in which case
.Xr netstart 8
reads the hostname files and runs
.Nm
for each interface that is to be configured via DHCP.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl c Ar file
Specify an alternate location to
.Pa /etc/dhclient.conf
for the configuration file.
.It Fl d
Do not daemonize.
If this option is specified,
.Nm
will run in the foreground and log to
.Em stderr .
.It Fl i Ar options
.Nm
will ignore any values provided by leases for the options specified.
This list will override any ignore statements in
.Xr dhclient.conf 5 .
.Ar options
must be a comma separated list of valid option names.
Invalid option names will cause the entire list to be discarded.
.It Fl L Ar file
Specify a
.Ar file
to write the option data to.
This causes
.Nm
to write two pseudo-leases,
.Dq offered
and
.Dq effective ,
to the specified file.
.Dq offered
will be the lease offered by the DHCP server;
.Dq effective
will be the modified lease bound to the interface.
.It Fl l Ar file
Specify an alternate location to
.Pa /var/db/dhclient.leases . Ns Aq Ar IFNAME
for the leases file.
.It Fl n
Configtest mode.
Only check the configuration file for validity.
.It Fl r
Release the current lease back to the server it came from.
.Nm
exits after removing the active lease from
.Pa /var/db/dhclient.leases . Ns Aq Ar IFNAME ,
deleting the address the lease caused to be added to the interface,
truncating any option file specified by
.Fl L
and sending a DHCPRELEASE packet to
the server that supplied the lease.
.Pp
If there is no
.Nm
controlling the specified interface, or
.Nm
has no active lease configured, no action is performed.
.It Fl v
Causes
.Nm
to log more information.
.Fl v
is implied if either
.Fl d
or
.Fl n
is present.
.El
.Pp
The DHCP protocol allows a host to contact a central server which
maintains a list of IP addresses which may be assigned on one or more
subnets.
A DHCP client may request an address from this pool, and
then use it on a temporary basis for communication on the network.
The DHCP protocol also provides a mechanism whereby a client can learn
important details about the network to which it is attached, such as
the location of a default router, the location of a name server, and
so on.
.Pp
On startup,
.Nm
reads
.Pa /etc/dhclient.conf
for configuration instructions.
It then attempts to configure the network interface
.Ar interface
with DHCP.
The special value
.Dq egress
may be used instead of a network interface name.
In this case
.Nm
will look for the network interface currently in the interface group
.Dq egress
and configure it with DHCP.
If there is more than one network interface in the egress group
.Nm
will exit with an error.
.Pp
When configuring the interface,
.Nm
attempts to remove any existing addresses, gateway routes that use
the interface, and non-permanent
.Xr arp 8
entries.
.Nm
automatically exits whenever a new
.Nm
is run on the same interface.
.Pp
Once the interface is configured,
.Nm
constructs a
.Xr resolv.conf 5
file.
It does this only when one or both of the options
.Cm domain-name
and
.Cm domain-name-servers
are present
(note that these options may be offered by the DHCP server but suppressed by
.Xr dhclient.conf 5 ) .
If a resolv.conf is constructed,
.Nm
appends any contents of the
.Xr resolv.conf.tail 5
file, which are read once at start up.
The constructed resolv.conf is copied into
.Pa /etc/resolv.conf
whenever the default route goes out the interface
.Nm
is running on.
.Nm
monitors the system for changes to the default route and re-checks
whether it should write its resolv.conf when possible changes are
detected.
.Pp
In order to keep track of leases across system reboots and server
restarts,
.Nm
keeps a list of leases it has been assigned in the
.Pa /var/db/dhclient.leases . Ns Aq Ar IFNAME
file.
.Ar IFNAME
represents the network interface of the DHCP client
.Pq e.g. em0 ,
one for each interface.
On startup, after reading the
.Xr dhclient.conf 5
file,
.Nm
reads the leases file to refresh its memory about what leases it has been
assigned.
.Pp
Old leases are kept around in case the DHCP server is unavailable when
.Nm
is first invoked (generally during the initial system boot
process).
In that event, old leases from the
.Pa dhclient.leases . Ns Aq Ar IFNAME
file which have not yet expired are tested, and if they are determined to
be valid, they are used until either they expire or the DHCP server
becomes available.
.Pp
A mobile host which may sometimes need to access a network on which no
DHCP server exists may be preloaded with a lease for a fixed
address on that network.
When all attempts to contact a DHCP server have failed,
.Nm
will try to validate the static lease, and if it
succeeds, it will use that lease until it is restarted.
.Pp
A mobile host may also travel to some networks on which DHCP is not
available but BOOTP is.
In that case, it may be advantageous to
arrange with the network administrator for an entry on the BOOTP
database, so that the host can boot quickly on that network rather
than cycling through the list of old leases.
.Sh FILES
.Bl -tag -width "/var/db/dhclient.leases.<IFNAME>XXX" -compact
.It Pa /etc/dhclient.conf
DHCP client configuration file
.It Pa /etc/hostname.XXX
interface-specific configuration files
.It Pa /var/db/dhclient.leases . Ns Aq Ar IFNAME
database of acquired leases
.El
.Sh SEE ALSO
.Xr dhclient.conf 5 ,
.Xr dhclient.leases 5 ,
.Xr hostname.if 5 ,
.Xr dhcpd 8 ,
.Xr dhcrelay 8
.Sh STANDARDS
.Rs
.%A R. Droms
.%D October 1993
.%R RFC 1534
.%T Interoperation Between DHCP and BOOTP
.Re
.Pp
.Rs
.%A R. Droms
.%D March 1997
.%R RFC 2131
.%T Dynamic Host Configuration Protocol
.Re
.Pp
.Rs
.%A S. Alexander
.%A R. Droms
.%D March 1997
.%R RFC 2132
.%T DHCP Options and BOOTP Vendor Extensions
.Re
.Pp
.Rs
.%A T. Lemon
.%A S. Cheshire
.%D November 2002
.%R RFC 3396
.%T Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)
.Re
.Pp
.Rs
.%A T. Lemon
.%A S. Cheshire
.%A B. Volz
.%D December 2002
.%R RFC 3442
.%T The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
.Re
.Pp
.Rs
.%A N. Swamy
.%A G. Halwasia
.%A P. Jhingram
.%D January 2013
.%R RFC 6842
.%T Client Identifier Option in DHCP Server Replies
.Re
.Sh AUTHORS
.An -nosplit
.Nm
was written by
.An Ted Lemon Aq Mt mellon@fugue.com
and
.An Elliot Poger Aq Mt elliot@poger.com .
.Pp
The current implementation was reworked by
.An Henning Brauer Aq Mt henning@openbsd.org .